Eight Killing 八煞 as the name suggests is a standard textbook entry level for FengShui practitioners. Derived from SanHe School, it is a watered down version of YangGong method. It comes in many forms such as: -

Sitting Mountain 8 Killing - 坐山八煞8 Killing Yellow Stream - 八煞黄泉

However, one SanYuan School - QianKun GuoBao/LongMen BaDaJu 龍門八大局 / 乾坤國寶 also incorporated the same into their techniques. It is extremely strange that as a SanYuan version require to use a SanHe method unless within SanYuan there is SanHe and such secret has been interwoven so deeply that it requires a genuine master to enlighten?

Saying so, there is also a SanYuan version of 8 Killing forces called: -

The keyword is sitting mountain, meaning how your building sits. Genuine YangGong people will measure the Seat instead of the Sit.

The original method is to first determine the incoming Qi based on the BaGua 8 directions and the sitting of the house or grave must not be located at the specific animal sign according to the 24 mountains, as in accordance to the above poem. Failing so bloodshed will manifest.

By the same token, if you are born in a specific year according to the animal sign as specify as above, your personalized Killing Force is the direction as mentioned. For example, a person born in Pig year will have his Killing Force in the Xun location. In 8 Mansion method, this Pig year born person must not live in a Gua 4 house. Gua 4 is Xun sitting house.

The alternative method as procured in QianKun GuoBao/LongMen BaDaJu 龍門八大局 / 乾坤國寶, is to use the sitting Gua of the house must not face Sha or Killing structure at the specific animal sign according to the 24 mountains, as in accordance to the above poem. Failing so bloodshed will manifest. This is known as the direct killing forces.

Whereas the indirect killing forces depend on the early and later heaven configuration of the sitting Gua of the house to determine the specific results of bloodshed or otherwise material lost.

Example, Kan house must not see ShaQi in the location of Dragon (Chen) and Dog (Xu). Early Heaven of Kan is Dui. Dui killing forces is Snake (Si). So if there is ShaQi at Si location - there will be bloodshed. Later Heaven of Kan is in Kun. Kun killing forces is Rabbit (Mao). So if there is ShaQi at Mao - there will be robbery and material lost.

Also note that there is a hidden Dog marked in red. Olden day master purposefully drop one or two of their techniques so that they will remain relevant and always better then their disciples.

Sitting mountain deals with branch while Yellow Stream deals with Stem and Gua location. The caveat is there must be "Pulling Nose" Sha drawing Qi away from the Main Door. So the reference point is the House Facing or Door Facing.

Example, Kun facing must not have road or water flowing out at Geng or Ding mountains. Similarly, Geng/Ding facing must not have Kun water flowing outward.

Here, another contentious issue with diverting opinion is: -

Measuring using the Heaven Plate as it is used in SanHe for measuring water.

Measuring using only the Earth Plate as it is one method specify in the classic RuDiYan 入地眼 or entering Earth Eye Classics.

Strangely, RuDiYan 入地眼 has never advocated the use of Heaven Plate and it is believed that this classic predates YangGong and having roots in Huang ShiGong 黃石公.

For a genuine YangGong practitioner, they are less bother with this method.

Commentary: -This is a SanYuan version of 8 Killing Forces using the sitting as the reference point. Example property sits Gui cannot see broken structure at Shen, Wei and Xun mountain. Its origin remains illusive and some account to it having taken from the asterism.

Conclusion: -Illustrated here are just some of the mainstream 8 Killing theories. There are countless other sub variants liked XuanKong DaGua Killing Forces and so forth. For the genuine practitioner, these are just end-users' techniques. All complete sets of SanHe or SanYuan methods had such being incorporated into their applications notably from the text Heaven Jade Classics - 天玉经.